Make Focaccia Dough

1. Combine Dry Ingredients - In a large mixing bowl, mix together bread flour and salt. Set aside.
2. Make Yeast Mixture - In a measuring cup or bowl, combine together, warm water, yeast (instant or active dry yeast), extra-virgin olive oil and sugar.
3. Combine & Rest Pour the yeast mixture into the flour mixture and mix until combined. The dough will be very sticky. Cover with a kitchen towel and allow to sit on the counter for 20 minutes.
4. Stretch & Fold - Perform a total 4 sets of stretch and folds, every 20 minutes. Dip your fingertips into water (everytime) then pick up one side of the dough, stretch it upwards and then fold the dough over and onto itself. Give the bowl a quarter turn and repeat the stretch and fold technique with the remaining 3 sides. Please see photos in the body of the post as reference. Cover the bowl, let rest for 20 minutes and then repeat the process 3 more times, making a total of 4 times. The dough will become more elastic with each rest.
5. Overnight Ferment - After stretching and folding the dough 4 times, pour a tablespoon of olive oil on top of the dough. Gently pat the oil with your fingertips to cover the whole surface. Cover with plastic wrap and place in the fridge for 12 to 48 hours.

Make Caramelized Onions

1. Cook Onions - Add olive oil, chopped onions, 4 to 5 sprigs of fresh thyme (or 3/4 teaspoon of dry thyme), salt and pepper into a large skillet. Stirring occasionally, cook the onions over medium/low heat until soft and caramelized. It will take about 45 minutes to 1 hour. When you're happy with the cook, turn the heat off and stir in a splash of balsamic vinegar. Transfer to a bowl and place in the fridge overnight. See note below.

Assembly

1. Transfer and Proof - When you're ready to bake, coat a 9x13 inch (23x28 cm) metal baking tray with 2 to 3 tablespoons of olive oil. If may seem like a lot of oil, but the focaccia will soak it up and this will also help crisp up the bottom and sides.
2. Remove the dough from the fridge and perform a final stretch and fold, then transfer to your baking tray. Gently stretch it to cover most of the bottom of the pan. It's okay if the dough contracts as you're doing this because it's cold. The dough will eventually fill out in the pan as it proofs. Let the dough double in size at room temperature for 1.5 to 2 hours.
3. Preheat Oven - With 20 minutes left of proofing, positions a rack in the center of the oven and preheat it to 425ºF (218ºC).
4. Add Toppings - After the dough doubles in size it should be very jiggly and have many bubbles. Pour 1 to 2 tablespoons of olive oil on top, followed by some fresh thyme and the caramelized onions.
5. Dimple the Dough - Now the fun part! Oil your fingertips and then dimple the dough. As you're doing this, the dough will create large bubbles. Try not to pop them. If you wish, you can top with a little more olive oil and flaky sea salt.
6. Bake - Bake for 25 to 30 minutes. After 20 minutes, loosely tent the top of the focaccia with foil to prevent the top from burning. It's ready when the top and edges are golden brown.
7. Cool & Serve - As soon as it comes out of the oven, let it sit in the tray for a minute or two and then transfer the focaccia onto a wire rack to cool. This will prevent the bottom from getting soggy. Cool for at least 15 minutes before serving. Enjoy!